Who is The Last Knight?

So we have a trailer for the 5th Transformers movie helmed by Mr Transformers Michael Bay. The trailer gives little about the plot but hints that another secret about earth and why the Transformers continue to return to Earth.

The key moment is the end shot where Optimus Prime takes down an attacking Bumblebee at which point he then asks for forgiveness as he stabs a blade from his arm into his friend. Is this a fake out or a key plot point?

We expect The Last Knight to be more of the same - giant robots beating the shit out of each other and we expect nothing else.

We're kind of hanging out to see Hot Rod (he better be as cool as he was in the cartoon movie) and also to find out what happened to Prime when he flew off into space at the end of Transformers 4. And of course, solve the incomprehensible mystery of why King Arthur appears to be in a Transformers movie.